Arrange the following compounds in the increasing order of their reactivity towards nucleophilic addition reactions.
(i) Ethanal, propanal, propanone, butanone
(ii) Benzaldehyde, p-tolualdehyde, p-nitrobenzaldehyde, acetophenone
(i) Butanone < propanone < propanal < ethanal
As we move from ethanal to butanone, + I-effect of alkyl group increases. The electron density on the carbon atom progressively increases and, hence attack by nucleophile becomes slower. Further, due to increase in the size of bulkier alkyl groups from ethanal to butanone, the attack of nucleophile is also hindered.
(ii) Acetophenone < p-tolualdehyde < benzaldehyde < p-nitrobenzaldehyde
Acetophenone is a ketone and all are aromatic aldehyde so, it is least reactive, p-tolualdehyde is less reactive than benzaldehyde due to hyperconjugation effect of group at p-position which increases the electron density on C-atom of carbonyl group, p-nitrobenzaldehyde is most reactive because the group is a powerful electron withdrawing group which withdraws electrons by —I and resonance effect thereby decreasing the electron density on the carbonyl carbon and making it more reactive than benzaldehyde